Natural gas, a source of hydrogen fuel, is a colorless and odorless combustible gas, and it provides an abundant fuel source that is commonly used in stationary fuel cell applications. Practical uses of fuel cells range from stationary industrial power supplies to portable power for consumer electronics. In many parts of the world, natural gas and synthetic natural gas (collectively, “natural gas”) are made available through an extensive pipeline distribution network that delivers the fuel to homes and businesses, including fuel cell manufacturers. Any leak in this pipeline network may pose a significant risk of gas accumulating to explosive levels without detection.
To reduce this risk, odorants are added to the natural gas which allows individuals to detect natural gas leaks without any equipment. The most common odorants used are sulfur compounds that have odors similar to rotten eggs or cabbage. These compounds vary by region, but typical examples include: mercaptans, sulfides, disulfides, thiophenic compounds and other organic or inorganic sulfur compounds. Problematically, the sulfur in such sulfur-containing compounds is a poison to most reforming and low temperature shift catalysts commonly found upstream of fuel cells, as well as most fuel cells themselves. Thus, it is necessary to remove the sulfur odorants at a point in the process before the natural gas is used as fuel for the fuel cell.
Two types of desulfurization are commonly used for fuel cell applications: ambient temperature and hot (typically 200-400° C.) desulfurization. In ambient temperature desulfurization, absorbents such as zeolites are often used to remove sulfur compounds primarily by physisorption. Chemisorption can occur on zeolitic materials for some compounds, such as hydrogen sulfide (H2S), but to a lesser extent compared to the physisorption. As these zeolitic materials remove compounds primarily based on size and shape of the molecules, they also remove hydrocarbons from the gas stream in addition to the sulfur-containing compounds. This is of little consequence for most of the hydrocarbon compounds, with the noted exception of benzene. This is a particular concern in many countries, including in the U.S., where the spent absorbent must be disposed as hazardous waste if the absorbent leaches >0.5 mg benzene/L of leachate as defined by the Environmental Protection Agency's (EPA) Toxicity Characteristic Leaching Procedure (TCLP). For fuel cell applications, the benzene not captured by the absorbent is normally reformed or oxidized depending on the application. Thus, only the benzene trapped on the absorbent is of concern.
Accordingly, present embodiments improve processes for desulfurization of natural gas, including ambient temperature desulfurization, which include those processes that use conventional zeolitic materials as absorbents. The improvements lower the level of leachable benzene at end of life (EOL) of the absorbent to <0.5 mg benzene/L leachate, while retaining the majority of sulfur adsorbed onto the zeolites. In this regard, present embodiments utilize the differences in binding strength and binding mechanisms for the relevant sulfur compounds and benzene. For example, one prior study reported findings on nonaromatic and saturated aromatic organosulfur compounds adsorbed on ion-exchanged zeolites (including Cu—Y zeolite). Velu, Xiaoliang and Song in “Mechanistic Investigations on the Adsorption of Organic Sulfur Compounds Over Solid Adsorbents in the Adsorptive Desulfurization of Transportation Fuels”, Am. Chem. Soc., Div. Fuel Chem. 2003, 48(2), 693. The authors observed that the mechanism of adsorption tends to occur by metal sulfur (M-S) interactions, whereas benzene is adsorbed by weaker π-complexation. In particular, the study pointed out that non-saturated aromatic sulfur compounds such as thiophene tended to bind to the zeolites either by direct M-S interaction or π-complexation. A recognition that such differences exist between sulfur compounds and benzene has advantageously and unexpectedly led to strategies for blocking, displacing or otherwise removing a substantial amount of benzene from used or spent absorbents.